A sealing labyrinth, also known as a labyrinth seal, comprises a rotary portion having fins (or wipers) together with a stationary bore covered in a lining of abradable material, or in a honeycomb structure capable of withstanding high temperatures.
When the engine starts, the fins of the seal rub lightly against the lining, biting into it, and thus leading to minimum spacing. This clearance varies during different cycles depending on the expansion of the parts and on the natural flexibility of the moving portions.
The labyrinth wipers serve to provide aerodynamic sealing between enclosures containing air at different pressures. In general, they are situated on the rotor portion facing stator portions covered in the lining of abradable material. They are constituted mainly by “blades” of annular shape, that are continuous or segmented in the circumferential direction, being directed radially either inwards or outwards.
In particular, when they are of continuous form, the wipers are liable to come into contact with the stator under certain operating configurations. In order to avoid them being destroyed in such situations, stators are fitted with coverings that provide the interface and that are known as being “abradable”. Under such circumstances, the usual sequences of wiper penetration into the abradable lining consists in making a radial cut associated with axial movement (turning-type machining).
Above, it is assumed, that the labyrinth seal wipers are formed on the rotary portion or rotor, however there exist situations in which the labyrinth seal wipers are formed on the stationary portion or stator, with the facing abradable ring then being located on the rotor.
During these contacts between wipers and the facing abradable ring, there exist certain configurations that make the system constituted by the rotor and the stator unstable from a vibratory point of view.
FIG. 1 shows an embodiment in which an abradable ring is used in a labyrinth seal, being placed facing wipers. This embodiment concerns a circuit for ventilating a high-pressure turbine located downstream from a combustion chamber 106.
In particular, there can be seen a turbine 108 with its rotor wheel that is rotatable about an axis X-X′.
The rotor of the turbine 108 comprises a turbine disk 40 fitted with blades 42 and a web 44 located upstream from the disk 40. The disk 40 and the web 44 have respective upstream flanges 40a for the disk 40 and 44a for the web 44, enabling them to be fastened to the downstream end 46 of the downstream 48 of the high-pressure compressor driven by the rotor of the turbine 108.
This arrangement of the cooling circuit has three successive bleed labyrinths.
A first bleed labyrinth 60 is formed upstream from the enclosure 52 separating the web 44 from the chamber end wall and downstream from the enclosure 54 separating the downstream cone 48 of the high-pressure compressor from the inner casing 50 of the combustion chamber 106. This first bleed labyrinth 60 comprises wipers 47 formed on the downstream cone 48 and a ring 58 of abradable material mounted at the end of a web 56 that is secured to the inner casing 50.
A second bleed labyrinth 62 is situated under the injectors 64, downstream from the enclosure 52. This second bleed labyrinth 62 is made of wipers 44b of the web 44 and a ring of abradable material 64a mounted on the injectors 64.
The third bleed labyrinth 66 is situated above the injectors 64, and comprises three successive wipers 44c formed on an angled portion 44d of the web 44 together with an abradable sealing ring 68a mounted on the inner casing 68.
Below, in order to explain the present invention, reference is made solely to the first bleed labyrinth 60, however the explanations given can be applied in like manner to the second bleed labyrinth 62 and/or to the third bleed labyrinth 66.
When contact is made between the abradable ring 58 and the wipers 47, it can be understood that the downstream cone 48 may be subjected to high levels of stress of a vibratory kind, and that this can cause it to vibrate in one or more resonant modes. Under such circumstances, the level of vibration then increases very quickly, subjecting the rotor made up of moving parts connected to the downstream cone 48 to deformation at levels liable to exceed their endurance limits, which also leads to damage to the abradable ring 58 and potentially to damage to one or more of the parts of the rotor, or even to the rotor breaking.
As a general rule, this phenomenon is very brief, either because an external event puts an end to it (change of speed of rotation of the rotor, thermal transient, . . . ), or else because the resonant frequency of the damaged rotor is different, thereby taking the rotor and stator system out of tune.
In general, in order to limit such damage, a suitable abradable material is selected together with wipers of specific shape or some other number of wipers, or else the stiffness of the stator portion (web 56) that supports the ring 58 of abradable material is modified.